1. Field of the Invention
The present invention relates generally to an electrochemical element, and more particularly to an electrochemical sensing or detecting element which comprises an electrochemical cell using a planar solid electrolyte body, and an electrical heater layer disposed on one side of the electrochemical cell to form a laminar structure.
2. Related Art Statement
There have been known various electrochemical devices, each of which comprises an electrochemical element using a solid electrolyte body. Such electrochemical devices are used, for example, as oxygen sensors to detect the oxygen concentration of an exhaust gas from internal combustion engines of automotive vehicles. Typical examples of such oxygen sensors include an oxygen sensor which employs a mass of oxygen-ion conductive solid electrolyte such as a zirconia ceramic, to determine the oxygen concentration according to the principle of an oxygen concentration cell. Also known in the art are electrochemical devices or elements which are employed as sensors or detectors for hydrogen, nitrogen, carbon dioxide, etc. In recent years, there has been an increasing trend of using an electrochemical cell of laminar structure which comprises a planar solid electrolyte body and planar electrodes disposed in contact with a surface or surfaces of the planar body of solid electrolyte.
Also, it is required to provide such a laminar electrochemical cell with a suitable electrical heater for heating the electrodes and the solid electrolyte body to an elevated temperature, in order to assure accurate and reliable operation of the cell, even while the temperature of a measurement gas is relatively low. For example, it is known to form an electrical heater layer on at least one side of the planar solid electrolyte body. The heater layer includes a heater member having a heat-generating portion in the form of a serpentine or sinuous strip, which runs in meandering or zig-zag fashion to heat at least a portion of the electrochemical cell at which the electrodes are disposed.
In an electrochemical element incorporating such an electrical heater layer, however, the life of the heater member may be shortened due to overheating and consequent destruction or disconnection due to fusion of the heat-generating portion, in case where a relatively large power supply is used to energize the heat-generating portion, so as to maintain the electrochemical element at a sufficiently high operating temperature for reliable operation thereof. On the other hand, if the power supply to the heater layer is limited to an irreducible minimum to the extent necessary for maintaining the permissible lowest operating temperature of the electrochemical element or cell, the heater life may be prolonged, but there arises a problem of uneven temperature distribution at the portion of the electrochemical element which is heated by the heat-generating portion of the heater member. In this case, the insufficiently heated portion of the element is not able to perform the intended electrochemical function, which may lead to declined measuring or sensing accuracy of the electrochemical element. It is a common practice in the art that the heat-generating portion of the heater member is formed as a serpentine or sinuous strip having a constant width and a constant thickness over its entire length. In this arrangement, central parts of the heat-generating portion located in or adjacent to a central portion of a heating region accommodating the serpentine strip are less likely to dissipate heat or easy to retain or hold heat, and therefore tend to have higher temperatures, then peripheral parts of the heat-generating portion adjacent to the periphery of the heating region. Thus, it is recognized that the central parts of the heat-generating portion have a higher possibility of overheating and consequent disconnection or physical failure, than the peripheral parts.